Ruthenium transfer hydrogenation catalysts are known in the literature. These ruthenium catalysts all require specific ligands (e.g. phosphine, NHC-ligands, bipyridine ligands). The recycling behaviour of these ruthenium catalysts is not optimal. Their activity (Turnover frequency: TOF) diminishes with the increase of recycling-cycles and a high turnover number (TON) and thus a long life-cycle cannot be achieved.
Thus, ruthenium catalysts with improved characteristics are still needed. In particular, ruthenium catalysts which are readily accessible and with a long lifetime are highly desirable.
Ruthenium phenolate complexes have been reported in the literature. The synthesis of ruthenium phenolate complexes is described by Kondo et al. in Organometallics 2005, 24, 905-910. A ruthenium phenolate complex was prepared by reacting Ru(η6-COT)(dmfm)2 with phenol. The resulting complex was isolated.
Panichakul et al., Organometallics 2008, 27, 6390-6392 describes the synthesis of BINOLate complexes of ruthenium. Said BINOLate complexes were prepared by reacting [RuCl2-p-cymene]2 with BINOL and isolated.
However, said ruthenium phenolate complexes are neither readily accessible nor has their use as catalysts been described.
Koelle et al., Organometallics 1991, 10, 2573-2577 describes the synthesis and structure of a bis(phenol) adduct of Cp*Ru(η5-oxocyclohexadienyl) (Cp*=η5-C5Me5). Catalytic properties have not been described.
Bhattacharya et al., J. Am. Chem. Soc. 1990, 112, 1088-1096 describes the synthesis of tris(3,5-di-tert-butylquinone) complexes of ruthenium. Their periodic trends in charge distribution have been investigated. Catalytic properties have not been described.
Yildiz et al., Asian Journal of Chemistry 2009, 21 (5), 4047-4053 describes Ru(III) complex compounds of alizarin. The use of these complexes as UV absorbers was investigated. Catalytic properties have not been described.
Treibs et al, Ber. Dtsch. Chem. Ges. 1927, 60B, 2335-2341, describes the gas phase synthesis of menthone starting from isopulegol by using a copper catalyst. Under these conditions significant amounts of thymol (35%) have been observed. Menthone was obtained as a not clearly identified mixture of L-menthone and D-isomenthone. Ni-catalysts, which may be used for the dehydrogenation of menthol to menthone, lead to the elimination of water from isopulegol.
The problem to be addressed by the present invention is to provide a catalyst, suitable for transfer hydrogenation reactions, which is readily accessible, preferably obtainable from a commercially available starting material, which can be used in low concentrations and having a long lifetime.
It is a further object of the invention to provide a catalyst suitable for transfer hydrogenation reactions, which lead to high selectivity and high yield of the transfer hydrogenation product.
It is a further object of the invention to provide an improved method for preparing menthone.